The present invention relates to a water based, cold seal, cohesive coating for flexible packaging systems, and more particularly to a water based, cold seal, cohesive coating containing a non-self-crosslinking acrylic emulsion for use in food packaging and especially snack food packaging.
A cold seal adhesive, also referred to as a cohesive coating, is a type of pressure sensitive adhesive which possesses the ability to form a strong bond to itself when pressure is applied and also to form a bond with the flexible substrate on which it is applied. Cohesive coatings are thus pressure respondent, i.e. the introduction of mechanical energy is all that is required to initiate seal formation. Cold seal adhesives also preferably lack adhesion to the surfaces such as plastic films and overprint varnishes which are on the opposite face of these flexible substrates so that such flexible substrates or webs coated with the cold seal adhesive may be stored in roll form without blocking, i.e. without sticking together. Cohesive coatings are used in a variety of different types of applications, but are particularly desirable for use as sealants in packaging for snack food such as candies, chocolates, ice cream and the like which are sensitive to heat thus rendering undesirable the use of heat sealable adhesives to seal such packages.
Typical cold seal adhesives have been formulated by combining a natural rubber elastomer, particularly a latex, with other ingredients such as stabilizers, antioxidants, and the like. Natural rubber elastomers exhibit most of the desirable cold seal properties noted above. Typically, these natural rubber elastomers where blended together with an acrylic emulsion. The acrylic emulsion provides advantageous properties to the natural rubber latex to enable the latex to adequately bond to flexible packaging substrates that may be covered with various types of inks, primers, release modifiers, slip agents and the like. Unfortunately, acrylic emulsions used in the past had heat reactive sites in the polymer chain which allow crosslinking of the polymer at temperatures ranging from above about 127° C. (260° F.). Crosslinking results in an improvement of some physical properties and in better water and solvent resistance, but also makes the acrylic more brittle so that its adhesive properties are reduced. In the past, the heat reactive functionality of such acrylic emulsions was not important since there was no way for the crosslinking to be initiated, i.e. processing temperatures of the flexible packaging substrates were well below the critical temperature of 127° C. (260° F.), and there were no known chemical initiators in the packaging materials per se. Thus, the acrylic emulsion was adapted for use in cohesive or cold seal coatings for flexible food packaging systems.
In more recent years, however, new inks such as acrylic based and polyurethane based inks have been introduced for use on flexible packaging substrates. Also, the substrates themselves are typically laminations of various types of films bonded together with new laminating adhesives such as water based polyurethane dispersions (PUD's) and acrylics to form the substrate lamination. Further, the substrate itself is being manufactured from new films with multiple co-extrusion layers such as high barrier resins, high surface energy resins and low temperature sealing resins for the purpose of promoting adhesion of cold seal or cohesive coatings to the surface of the substrates. In addition, new processing equipment has been developed to speed the rate of converting raw materials to the flexible end product packaging assembly. New “extreme dryers” are being used to superheat and compress oven air that is used to dry flexible packaging coatings, and for example these extreme dryers run at 135° C. (275° F.) at 2.1 kg/cm2 (30 psi). Unfortunately, running at such temperatures results in being 15 degrees higher than the critical temperature of 127° C. (260° F.) mentioned above for initiating crosslinking. As a result, the acrylic latex used in the past crosslinked with the result that although it became tougher, it also decreased its cohesiveness resulting in what is commonly referred to in the art as “seal deadening.” Seal deadening of a flexible package results in failure of the seal which is unacceptable to manufacturers and consumers. As a result, it was desirable to develop a water based cohesive which could be utilized with flexible packaging systems that are manufactured by current techniques so that such systems do not result in cohesive failure.